Human serum enzyme assays for clinical diagnosis usually make use of measurements of catalytic activity. In many situations, this may not be ideal; e.g. isoenzymes reflecting different damaged tissues may be indistinguishable or inactive enzymes may be released. Radioimmunoassay offers a very sensitive and specific way in which to measure enzyme concentration and to distinguish isoenzymes. Creatine kinase was chosen as a model for this approach because of its clinical importance and its simple isoenzyme distribution. The enzymes from human skeletal muscle and brain have been purified, used as immunogens to raise antibodies, and labeled with 125I. Radioimmunoassays for each of these enzymes have been developed and are now being applied in clinical diagnostic studies.